Glass optical waveguide filaments are remarkably strong as they are drawn, but their strength is greatly affected by physical handling. They must therefore be coated with a protective material before they come in contact with the filament drawing tractors. For a number of reasons including improved diameter control and cleanliness, the tractor should be as close as possible to the draw furnace. High silica content filaments are drawn at temperatures as high as 1850.degree. C. At a drawing speed of 0.5 m/sec the temperature of a 125 .mu.m diameter filament is about 600.degree. C. at a distance of 10 cm from the furnace. It has been possible to apply acceptable coatings to filaments formed at such relatively low drawing speeds.
The economical production of large quantities of optical waveguide filaments will undoubtedly require filament drawing speeds greater than 1 m/sec. As filament drawing speeds approach 1 m/sec, presently constituted filament drawing systems do not afford sufficient time for the filament to cool by natural processes to temperatures which are compatible with application techniques employing presently developed waveguide coating materials. Thus, if a coating is applied from a solvent-containing solution, the hot filament can cause flash evaporation of the solvent vehicle which in turn can cause the resulting coating to be very rough and noncontinuous. If the coating is applied from a 100% solids solution, the hot filament can cause the coating material to set up in the coating apparatus around the filament, thus preventing any further coating material from being applied to the filament.
In a prior filament coating apparatus the filament was contacted by a reinforced felt wick prior to entering the coating tank. This wick allowed the application of a primer precoat to the filament, and it held the filament in the window of an optical micrometer which was employed for monitoring filament diameter. As filament drawing speeds were increased, the temperature of the filament at the wick also increased, thereby causing it to become dry and abrasive to the filament.